Need of Inspection (Metrology)

1.2.
In order to determine the fitness of anything made, man has always used inspection. But
industrial inspection is of recent origin and has scientific approach behind it. It came into being
because of mass production which involved interchangeability of parts. In old craft, same craftsman
used to be producer as well as assembler. Separate inspections were not required. If any component
part did not fit properly at the time of assembly, the craftsman would make the necessary
adjustments in either of the mating parts so that each assembly functioned properly. So actually
speaking, no two parts will be alike/and there was practically no reason why they should be.
Now new production techniques have been developed and parts are being manufactured in
large scale due to low-cost methods of mass production. So hand-fit methods cannot serve the
purpose any more. When large number of components of same part are being produced, then any
part would be required to fit properly into any other mating component part. This required
specialisation of men and machines for the performance of certain operations. It has, therefore,
been considered necessary to divorce the worker from all round crafts work and to supplant hand-fit
methods with interchangeable manufacture.
The modern production techniques require that production of complete article be broken up
into various component parts so that the production of each component part becomes an independent
process. The various parts to be assembled together in assembly shop come from various shops.
Rather some parts are manufactured in other factories also and then assembled at one place. So it
is very essential that parts must be so fabricated that the satisfactory mating of any pair chosen
at random is possible. In order that this may be possible, the dimensions of the component part
must be confined within the prescribed limits which are such as to permit the assembly with a
predetermined fit. Thus industrial inspection assumed its importance due to necessity of suitable
mating of various components manufactured separately. It may be appreciated that when large
quantities of work-pieces are manufactured on the basis of interchangeability, it is not necessary
to actually measure the important features and much time could be saved by using gauges which
determine whether or not a particular feature is within the prescribed limits. The methods of
gauging, therefore, determine the dimensional accuracy of a feature, without reference to its actual
size.
The purpose of dimensional control is however not to strive for the exact size as it is
impossible to produce all the parts of exactly same size due to so many inherent and random sources
of errors in machines and men. The principal aim is to control and restrict the variations within
the prescribed limits. Since we are interested in producing the parts such that assembly meets the
prescribed work standard, we must not aim at accuracy beyond the set limits which, otherwise is
likely to lead to wastage of time and uneconomical results.
Lastly, inspection led to development of precision inspection instruments which caused the
transition from crude machines to better designed and precision machines. It had also led to
improvements in metallurgy and raw material manufacturing due to demands of high accuracy and
precision. Inspection has also introduced a spirit of competition and led to production of quality
products in volume by eliminating tooling bottle-necks and better processing techniques.1.2.1.

Measuring Means.

The means of measurements could be classified as follows :(i) Standards (Reference masters or setting standards)—These are used to reproduce one
or several definite values of a given quantity.(it) Fixed gauges—These are used to check the dimensions, form, and position of product
features.(iii) Measuring instruments—These are used to determine the values of the measured
quantity.